A major advantage of phased array antennas is their ability to steer the beam electronically, eliminating the need for mechanical pointing and alignment. Another benefit is that the beam steering can be performed quickly, which allows tracking of rapidly moving targets, and tracking of multiple targets. The rapid beam steering also facilitates applications where an antenna on a moving platform (e.g. a ship at sea) it to maintain contact with a fixed entity such as a communications or broadcast satellite.
A common application of phased array antennas is in the implementation of radar systems, especially synthetic aperture radar systems.
Radio detection and ranging, or radar as it is commonly known, has been in existence since World War II and is used for a wide variety of applications. For example, radars are used for tracking the position of objects such as airplanes, ships and other vehicles or monitoring atmospheric conditions. Imaging radars have been developed for constructing images of terrain or objects.
Basic radar systems operate by transmitting a radio frequency signal, usually in the form of a short pulse at a target. A basic radar system is limited in both range resolution and azimuth resolution. Various techniques have been developed to overcome the limitations of a basic radar system. For example, to improve range resolution techniques such as pulse compression can be used.
To improve azimuth resolution without requiring an unacceptably large antenna, the Synthetic Aperture Radar technique has been developed. Synthetic Aperture Radars are now commonly used in both airborne and spaceborne (e.g. an airplane or satellite) based applications.
Modern Synthetic Aperture Radar systems require operational flexibility by supporting imaging over a wide range of resolutions and image swath widths. This operational flexibility requires the use of an active phased array antenna system.
Current active phased array systems for spaceborne applications suffer from a number of limitations, which restricts their broader use. The antennas are relatively large, on the order of 10 to 20 meters in length, and 1 to 2 meters in width. To preserve the quality of the beam and maintain it stable requires that the antenna itself be rigid and that it be rigidly supported to keep the antenna flat within the required tolerances. This results in an antenna with a high mass and requires support trusses or other mechanical means to provide the required stiffness when extended.
The size of the antenna generally prohibits launching the antennas in their operational configuration, as it is too large to fit within the available payload volume of the launch vehicle. The antenna is to be folded and stowed for launch, then deployed once in orbit. Complicated and expensive mechanisms to deploy the antenna and hold it rigid when deployed are to be specially designed. Special purpose mechanisms may also be designed and constructed to securely hold the antenna panels while stowed during launch and ensure that that the antenna is not damaged by the stresses incurred during launch. The high mass of the antenna makes the task of stowing and deploying it much more difficult.
The elements of the active phased array require a complex set of interconnections between the main bus structure and the antenna elements. Connections are needed for power, control, monitoring and distribution of radio-frequency signals for both transmit and receive. Complicated azimuth and elevation beam forming devices and interconnects are required. These interconnections further add to the overall mass, complexity and cost of the antenna. In addition, the interconnections may be made to bridge the hinges between the panels of the antenna adding to the manufacturing complexity and cost, and reducing the overall reliability.
The RADARSAT-2 spacecraft is an example of a state-of-the-art Synthetic Aperture Radar System using an active phased array antenna. The antenna in this instance is 15 meters in length and 1.5 meters in width. It consists of two wings, each containing 2 panels with each panel approximately 3.75 meters in length and 1.5 meters in width. Each panel contains 4 columns with each column containing 32 transmit/receive modules each with an associated sub-array with 20 radiating elements. A total of 512 transmit receive modules are used in the antenna. The overall mass of the antenna is approximately 785 kg. The extendible support structure required to deploy the antenna panels and maintain them in place has a mass of approximately 120 kg. The mechanisms used to hold the antenna while stowed, and then release it for deployment, add an additional approximately 120 kg of mass. The total mass required by the antenna is approximately 1025 kg. This large mass in turn drives the design of the spacecraft bus structure and attitude control systems, resulting in a larger, heavier spacecraft.
The large mass and complex design mean that the overall cost of designing, building and launching this class of spacecraft is high. This restricts the use of this technology to specialized applications and limits the number of spacecraft that can be launched, reducing the frequency of observation and limiting the operational missions that can be supported.